Filters are important Microwave components required to transmit signals within a desired frequency band and to filter out signals beyond the desired frequency band. Generally a frequency band within which signals can pass through a filter is called the pass-band, and a frequency band within which signals are filtered out by the filter is called the cut-off region. An ideal filter can transmit signals in a pass-band without attenuation, and cause signals in the cut-off region to attenuate infinitely. To achieve the above effect, the transition between pass-band and cut-off region should be as steep as possible, namely the pass-band edges should be as steep as it could be. Commonly, poles of the filter (the amount of resonator) can be added to increase the steepness of the pass-band edges, but this will bring distinct insertion losses, causing the attenuation of the pass-band to become larger and exacerbating the performance of the filter. So a normal microstrip filter with more poles has a larger insertion loss, which is difficult to meet the needs in the fields of high standard requirements, such as satellite applications. In this instance only wave-guide filter can be applied to achieve the requirements.
Recently, with the development of techniques of preparation for HTS (high temperature superconductive) materials, including preparation for single crystal and thin film etc, it is possible for superconductive microstrip to be used in practical applications. Comparing with common microstrip filters, a superconductive microstrip filter has lower insertion loss, better anti-interference ability against neighbor frequency, higher Q value of the resonator (below 10 GHz, Q value is about 40,000-100,000). Experiment results show that a superconductive microstrip filter has steeper band-edges, extremely low insertion loss and flat pass-band characteristic, which is close to the ideal filter in performance. A superconductive microstrip filter also has the merit of smaller volume and lighter weight as compared with common microstrip filter. With the above characteristics, superconductive microstrip filters, instead of wave-guide filters, shall be employed in fields having higher requirements for filters.
FIG. 1 shows an superconductive microstrip filter invented in England in 2000, which comprises 8 open-loop form resonators in the same or similar size, having a substrate of LaAIO.sub.3, wherein the total length of the filter is 39 mm and the total width of the filter is 23.5 mm. As shown in FIG. 1, in this superconductive microstrip filter, resonator 1, 2, 3, 4, 5, 6, 7, and 8 are disturbed in an axis symmetric configuration, the intervals between the resonators are determined by requirements for the performance of the microstrip filter. Each resonator is made of an superconductive microstrip line which is folded like a ring structure with a gap of width Wg, the total length of the ring structure microstrip line is about a half of the wavelength corresponding to the center frequency of superconductive microstrip filter. Here, the electric field is mostly concentrated at the gap of the ring structure, so this part of the resonator is like a capacitance; the magnetic field is mostly disturbed on the other side of the resonator opposite to the gap, so the superconductive microstrip line functions similarly to an inductance. The width WO of the input feed-line 1 and output feed-line 12 corresponds to 50.OMEGA. of input impedance and output impedance. Because the lengths of the input feed-line 11 and output feed-line 12 have no influence on the filter performance, the respective lengths could be several millimeters in accordance with technique requirements. The positions at which the input feed-line 11 and output feed-line 12 are connected to neighboring resonator 1 and 8 are determined by input and output impedance matching.
FIG. 2 shows the frequency response of the superconductive microstrip filter in FIG. 1 at 55K when combined with a LNA (low noise amplifier). In FIG. 2, solid line 21 indicates the characteristic curve of transmission loss of the superconductive microstrip filter, dash line 22 represents characteristic curve of reflect loss of the superconductive microstrip filter. In FIG. 2, the X-axis “Frequency (MHz)” denotes the frequency of the signals in mega-Hertz. S11 and S21 denote the transmission loss values of 21 and 22. It can be seen from the figure, the insertion loss of the filter is about 0.13 dB at pass-band, the steepness of the low band-edge is 20 dB/MHz, the steepness of the high band-edge is 15 dB/MHz. While this type of superconductive microstrip filter has high Q value, low insertion loss and good band-edge steepness, the resonators constituting the superconductive microstrip filter are too large to effectively use a substrate space, therefore the poles of the filter can not be increased by increasing the number of the resonators, whereas increasing the number of the resonators can substantially improve the steepness. Hence, the above described structure is not satisfying
In order to overcome shortages of the existing techniques, it is necessary to advance a resonator of smaller dimension to increase the number of resonators within the limited substrate space of a superconductive microstrip filter.